The sleeve, which is normally mounted on a carriage, usually performs linear movements along three co-ordinate axes and has the machining tool mounted on its end. Depending on the configuration of the numerical control machine, the sleeve may be vertical or horizontal.
In the first configuration, which is characteristic of so-called gantry machines, the machine comprises a base or bench having two vertical walls or shoulders which support a cross-beam which has, running along it, a support structure, called a carriage, to which the sleeve is integrally joined and movable vertically. In the second type of machine, a vertical upright, which is slidable or fixed onto the base, extends upwards from the base or bench. The spindle-support carriage is mounted on the aforementioned upright, with the possibility of sliding in the vertical direction, while the sleeve, which is arranged horizontally and associated with the carriage, is movable horizontally.
The sleeve is per se a substantially tubular structure which is usually made of welded steel and inside which the main component of the machine tool, or the so-called spindle, is seated. There are two types of spindles: the first type, called an electric spindle, is positioned at the free end of the sleeve and has an attachment for the machining tool; the second type, called a motorised spindle, is housed inside the sleeve itself. For the sake of simplicity, in the present invention, the term “spindle” will be solely used since the sleeve which forms the invention may be used for supporting both an electric spindle and a motorised spindle.
A machine tool is subject to considerable stresses which may cause the deformation of some components, in particular the sleeve, which deformation adversely affects the precision required during machining, such as for example the machining of moulds, in which the tolerances of the finished product must be very small. If we consider the sleeve in particular, it must satisfy varying requirements which also are of a mutually opposing nature and can be summarised principally as follows:
a) rigidity, which by counteracting the abovementioned deformations, ensures that the requirements for precision during machining are satisfied;
b) damping of the vibrations resulting from machining, said vibrations also adversely affecting the machining precision and the possibility of achieving the desired tolerances; and
c) dissipation of the heat which is produced by the spindle during operation of the machine tool and is due both to the friction (for example of the bearings and the sliding guides) and to the various functional electric components (such as the motors, for example) as a result of the Joule effect.
In order to satisfy the requirement for rigidity, the sleeve is usually made of welded steel or cast iron with a considerable thickness, which however also results in a notable weight. The latter affects the remaining structural part of the machine which must be especially strengthened. Apart from this (which in any case constitutes a drawback affecting the design and manufacture of the machine), there is an increase in the masses involved and therefore the sleeve movement times, unless sleeve actuating motors are used which are more powerful—and therefore ultimately more costly.
The use of steel for construction of the sleeves in any case does not ensure a good level of damping of the vibrations, while the use of nodular cast iron, which is able to dampen vibrations, has the drawback of a low modulus of elasticity of this material.
Finally, there remains the unsolved problem of dissipating the heat generated during operation of the machine.